1. Field of the Invention
Present invention relates to a method and apparatus for surface processing of a sintered bearing that retains oil and particularly relates to said method and apparatus which enable improvements of the efficiency of covering the blow holes and the surface roughness of protuberances that are poreless surfaces by way of making the inner diameter surface to be surface processed by a correction rod, where many correction grooves have been formulated at the top part, at least in the sizing process.
2. Description of the Prior Art
Generally, a sintered oilless bearing is of the type introduced in Japan Patent Publication Hei No. 5-180229, in which a bearing has a shape as in FIG. 1, where many protuberances 1a and grooves 1b have been formed on the inner circumferential surface.
Such a sintered oilless bearing 1 is made to have porosity, as a whole, by way of a sintering process, after compression molding in a definite frame, in which state the bearing is manufactured by permeating oil.
Sintered oilless bearing 1 manufactured by the above method constitutes a bearing unit retained by forced fit insertion in a bearing housing. If a shaft is inserted in the inner diameter part of a sintered oilless bearing 1 retained in the bearing housing, oil is emitted from each groove 1b of the sintered oilless bearing 1 at the time of shaft rotation. If this oil is coated on the inner surface of protuberance 1a, the shaft makes sliding contact with the oil film formed thereby, so that the shaft is smoothly driven.
Meanwhile, oil permeation in the sintered oilless bearing 1 is under the sintered condition of the sintered oilless bearing 1, before which oil permeation, a sizing pricess is performed that processes the inner surface of the bearing.
This sizing process is performed by a separate sizing tool that reciprocates on the inner surface of protuberance 1 a so that the surface is smoothed and the process carries out the function so that pores formed on the surface are blocked at the same time.
If the sintered oilless bearing is processed on its surface by such sizing process, the oil emitted out of the groove when the shaft revolves in the motor is coated on the inner diameter surface with protuberance 1a so that an oil film of definite thickness is formed so as to substantially reduce the frictional force with the shaft and to allow smooth driving.
The present applicant has already proposed a correction device and method that allow processing of the inner surface with protuberances 1a that are formed at sintered oilless bearing 1, in Japan Patent Registration No. 2912599.
Namely, as in FIG. 2, a correcting rod 2 is furnished that is integrally formed by an upper part of the protuberance correction part 2a, a groove formation part 2b having a larger outer diameter than this protuberance correction part 2a, and a protuberance formation part 2c having a smaller outer diameter than the protuberance correction part 2a, by inserting the outer circumferential surface of the connection part radially between the protuberance correction part 2a and groove formation part 2b so that the inner diameter surface of protuberance 1a may be surface processed using the above furnished correcting rod 2.
When it made in bearing 1, just after sintering, the outer diameter of protuberance correction part 2a is equal to the inner diameter of protuberance 1a, and the outer diameter of groove formation part 2b is equal to the outer diameter of groove 1b, while the outer diameter of protuberance formation part 2c is less than the inner diameter of protuberance 1a. 
In the prior invention, there were furnished, together with the above correcting rod 2, upper and lower punches and a guide frame that guides so that the upper and lower punches may slide up and down for inner diameter surface processing of the sintered oilless bearing 1.
If bearing 1 is pushed from above by upper punch 3 using such correction device as in FIG. 3, bearing 1 is compressed, from above and below, between upper punch 3 and lower punch 4, because the bottom surface of bearing 1 is supported by lower punch 4, so that the thickness of bearing 1 is decreased, in fine degree, at this time, and the inner diameter of protuberance 1a is decreased and eventually, it contacts closely with outer the diameter surface of protuberance formation part 2c so as to be equal in diameter with protuberance formation part 2c. 
If upper punch 3 is moved upward in this compressed state so as to be out of guide frame 5 and also lower punch 4 is moved upward, bearing 1, seated on lower punch 4, is moved upward at same time.
If bearing 1 is thus moved upward, it passes protuberance correction part 2a, because correcting rod 2 remains fixed, so surface processing takes place, as inner the diameter surface of protuberance 1a, with reduced inner diameter of bearing 1, passes protuberance correction part 2a, having larger outside diameter than the inner diameter surface.
Namely, because the inner diameter surface of protuberance 1a closely contacts the outer diameter surface of protuberance formation part 2c owing to compression by upper punch 4, it has a diameter equal to the outer diameter of protuberance formation part 2c. While protuberance correction part 2a is made to have a larger outer diameter than protuberance formation part 2c, the inner diameter surface of protuberance 1a is expanded to have an inner diameter equal to the diameter of protuberance correction part 2a, as protuberance 1a passes protuberance correction part 2a. 
Such inner diameter expansion takes place by forcibly pushing the inner diameter surface of protuberance 1a, whereby the inner diameter surface of protuberance 1a, having a rough surface by sintering, becomes smoother as numerous blowholes formed on the surface are closed.
Thus, if lower punch 4 is moved upward to pass through protuberance correction part 2a of correcting rod 2 so as to completely clear correcting rod 2, we are furnished with a sintered oilless bearing 1 that is surface processed, to be used by assembling at bearing holder after permeating the oil.
If sintered oilless bearing 1 is surface processed by the above correction device, concentricity between protuberance 1a and groove 1b is accurately restored, which was lost during sintering process of sintered oilless bearing 1, and the surface friction with the shaft contacting the inner diameter surface of protuberance 1a may be reduced.
However, protuberance correction part 2a in correcting rod 2 makes the outer circumferential surface to be simply planar in previous invention. Although there is merit in that inner diameter surface roughness is somewhat improved by inner diameter surface processing using such correcting rod 2, there is a disadvantage in that blowhole covering of the porous surface, which is the most important, is not accomplished properly.
If porous surface blowholes are not properly covered, there arises a problem that the oil film is not properly formed when the shaft revolves, because a part of the oil ejected from groove 1b is again absorbed in inner diameter surface of protuberance 1a while sintered oilless bearing 1 is being used, having been assembled as a bearing unit after permeating the oil as described above.
Because oil film formed on this inner diameter surface is a factor having the greatest influence on friction force with the shaft, friction surface wear is eventually accelerated by an intensified friction force between the shaft and the inner diameter surface of protuberance 1a, so that the useful life of the bearing is cut short, and the friction force acts as a load to the shaft drive force, at the same time, so that there is also another serious problem that the required drive force is not exerted.
Present invention is devised to correct the above problems and its main purpose is to improve oil film formation efficiency on the inner diameter surface by forming one or more correction grooves on the outer circumferential surface of the protuberance correction part in the correcting rod, so that blowholes formed at the inner diameter surface of protuberance would effectively be closed at the time of the sizing process by the correcting rod.
Another purpose of the present invention is to further improve the covering efficiency of blowholes by also performing surface processing at the time of assembling bearing unit by making one or more correction grooves to be formed on the outer circumferential surface of a mandrel during the process of assembling with the bearing holder, not only with the above surface processing during the sizing process.
The present invention, to achieve the above purposes, features a fabrication method comprising a step of compression molding a metal powder having been put in a definite frame, a step of sintering the molded bearing, a step of inserting the sintered bearing in the correcting rod so that the bearing is seated on the lower punch top in the guide frame, a step of compressing the bearing seated on the lower punch by an upper punch, a step of separating the upper punch from the guide frame by lifting the upper punch, a step of covering numerous blowholes formed at inner diameter surface as the inner diameter surface of the protuberance is expanded according to a plurality of correction grooves formed on the outer circumferential surface of the protuberance correction part, by lifting the lower punch so that the compressed bearing passes through the protuberance correction part of the correcting rod, a step of separating the surface processed bearing from the correcting rod by completely lifting up the lower punch, a step of permeating oil in the sintered oilless bearing that has been surface processed, and a step of assembling the sintered oilless bearing by forced fitting, having spacers laid on top and bottom of a bearing holder, using the upper and lower punches and mandrel.